Hot melt gaskets are solid at room temperature and molten at elevated temperatures (generally 250.degree. F. to 450.degree. F.). Hot melt gaskets have been used in closures, in particular plastic closures. Hot melt gaskets have several advantages over other gaskets such as solvent based gaskets, water based gaskets and plastisols. Hot melt gaskets are solvent free, do not require fluxing, heating or drying after application, are easy to apply and set up quickly. In view of these advantages, hot melt gaskets have obtained some acceptance in the container industry.
Generally, hot melt gaskets have been based upon thermoplastic block copolymers or ethylene vinyl acetate copolymers (EVA) or mixtures thereof. EVA copolymers are preferred due to cost, thermal stability and flexibility.
Present hot melt gaskets however have several drawbacks. In particular, hot melt gaskets are notorious for their poor adhesion to plastic closures formed from polypropylene. Part of the problem is the limited amount of tackifier resin which can be added to a hot melt gasket without degrading its sealing properties. Specifically, tackifier resins adversely effect the durometer of the hot melt gasket, increasing the durometer or hardness of the gasket thereby reducing its ability to seal. Additionally, tackifier resins have been shown to increase the removal torque of the applied closures, making them difficult or impossible to remove and therefore commercially unacceptable. This presents a serious problem to the widespread use of hot melt gaskets in the container industry as polypropylene is the preferred closure material.
To overcome this problem, various mechanical interlocking or hold fast devices have been incorporated into plastic closures to retain the hot melt gaskets. These devices include ribs, undercuts and retaining lips, recesses and hollows and raised obtrusions. All of these devices are designed to trap and hold the hot melt gasket in place. While generally successful, they require a reworking of the closure mold and often interfere with the removal of the closure from the mold. These problems have been countered with the use of multipiece molds or subsequent formation of the devices after molding or slowing down the molding process to allow for the proper removal of the closures. In doing so, however, the cost of molding these closures has risen dramatically. Further, these devices are not always suitable in a closure. They can interfere with the lining of the hot melt gasket and interfere with the application of the closure to the container. Lastly, these devices do not always ensure that the gasket will be retained, especially in screwed-on closures where the resistance between the gasket and the container surface upon application and removal can dislodge the gasket from these devices. The present invention provides a hot melt gasket and method of applying the same that overcomes these problems of adhesion and sealing.